1. Field of the Invention
The present invention relates to a device and a method for determining geometric measurement values of an eye. In particular, the invention relates to a device and a method for determining geometric measurement values of a human eye in which, by means of a light projector, a beam of rays is projected through a section of the eye, in which a first image of at least a partial region of the section illuminated by the light projector is registered, by means of image capturing means, at a first observation angle, from a first position outside the beam of rays, and in which a second image of the partial region is registered, by means of the image capturing means, at a second observation angle, from a second position outside the beam of rays.
2. Description of Related Art
Described in the printed patent specification U.S. Pat. No. 6,234,631 is a method for measuring the anterior and posterior corneal surfaces and the corneal thickness of the eye. In the method according to U.S. Pat. No. 6,234,631, a front-view camera is used to capture a front view, and, symmetrically thereto, a left and a right camera to capture two side views of the eye. The left and the right camera are each oriented at a 45-degree-angle to the optical axis of the front-view camera. In the method according to U.S. Pat. No. 6,234,631, a light pattern in the shape of a cross, similar to two simultaneously projected orthogonal slits, is projected onto the cornea, and the iris is illuminated with infrared light for contrasting the pupil. The horizontal portion of the light cross is captured with the front-view camera, and the vertical portion of the light cross is captured by each of the left and right cameras. At the same time an image of the pupil is captured by each of the three cameras. From the pupil images, the pupil contour is determined from the viewpoint of each of the three cameras. From the images of the light cross, and with assumption of an approximate corneal surface, a first approximate value is calculated for the corneal thickness on the basis of the ray trajectory light source‥cornea‥camera, by means of so-called ray tracing, according to U.S. Pat. No. 6,234,631. This first approximate value serves as an initial value for an iterative procedure in which, starting from the previously determined topography of the anterior corneal surface, the corneal thickness and the topography of the posterior corneal surface are determined. Determination of the topography of the anterior corneal surface takes place through iterative calculation from images of reflections of a placido disc on the cornea, which are taken by means of the three cameras. The corneal thickness and the topography of the posterior corneal surface are calculated, according to U.S. Pat. No. 6,234,631, iteratively on the basis of the ray trajectory pupil contour‥cornea‥camera (ray tracing), the views of each of the three cameras being taken into consideration.
Described in the published patent application WO 01/62140 is a system to measure the topography of both corneal surfaces and the corneal thickness. In the system according to WO 01/62140, a light beam, for example a laser beam, expanded in a fan shape by means of a cylindrical lens, is shone on the cornea. The system is designed such that the fan-shaped light beam can be rotated. The illuminated region in the area of intersection of the fan-shaped light beam and of the cornea is captured by two cameras, which are disposed at right angles to one another, so that their directions of observation include a 90 degree angle in the direction of view of the optical axis of the eye. The pictures taken by the cameras of the illuminated section are each undistorted only when the plane of the section lies perpendicular (seen in the direction of the optical axis of the eye) to the direction of observation of the respective camera. The other camera registers the section in this position from above, and thus can record neither the thickness nor the profile of the cornea. In the system according to WO 01/62140, a corrected, undistorted image is generated from the images of the two cameras, the undistorted image corresponding to that of a virtual rotating camera. The thickness of the cornea is then determined from the corrected image. Through rotation of the light source, the corneal topography can be put together from a multiplicity of corrected images.
For calculation of the corneal thickness by means of ray tracing, the surface inclination of the cornea must also be known, in addition to the refraction index of the cornea, the angle of illumination (directions of the projected light beams), and the observation angle (direction of the registered light beams). The refraction index can be assumed to be known, and both the illumination and the observation angles can be determined through calibration of the system according to WO 01/62140. For determination of the surface inclination, however, the system according to WO 01/62140 requires additional means if the influence of the surface inclination of the cornea is not supposed to falsify the result of the thickness calculation according to WO 01/62140.